This invention relates to a new and useful catalyst and to a new and useful process for the oxidation of an aromatic compound in the liquid phase with an oxygen-containing gas at reaction conditions wherein the said aromatic compound has an alkyl group attached to the aromatic ring, the alkyl group containing an alpha hydrogen.
In this new and useful process, aromatic compounds with an alkyl group having at least one alpha hydrogen, including aromatic compounds wherein the alkyl groups are unsubstituted and substituted, are contacted in the liquid phase at reaction conditions with an oxygen-containing gas in the presence of a catalyst which is prepared by milling or grinding an anhydrous transition metal halide with silver nitrite, the transition metal being selected from the Groups IVb to VIII, and Ib and IIb of the Periodic Table, wherein the mole ratios of metal halide to silver nitrite are in the ratio to 1:1 to 1:5.
More particularly, the invention teaches the preparation of useful compounds through the oxidation of alkyl aromatic hydrocarbons in the liquid phase by the use of a new catalyst, including the preparation of the new catalyst.
From an industrial viewpoint, the important reactions of alkyl aromatic compounds are those involving the oxidation of methyl and ethyl groups. A number of examples can be cited. The isomers of toluic acid are bacteriostats, have use as animal feed supplements, and are used in organic syntheses to form insect repellants are well as other products. The tolyl aldehydes are useful in perfumes and in dyestuff intermediates. Ethylbenzene is reacted by oxidative dehydrogenation to form styrene monomer. Styrene is a a monomer of great industrial significance in that it is the precursor for polystyrene and is used in styrene-butadiene rubber (SBR) and in acrylonitrile-butadiene-styrene (ABS) copolymers. Cumene is oxidized to alpha-methylstyrene, a polymerization monomer especially used in polyesters.
Accordingly, the oxidation of hydrocarbons has been extensively studied and have been the subject of many publications, including patents. Two approaches have, in general, been taken in these studies of the oxidation of hydrocarbons, namely, reactions involving liquid phase reactions and those involving gaseous phase reactions.
If oxidation reactions are carried out in the gaseous phase, temperature control is an extremely important factor since all oxidations are exothermic. Precise temperature control is necessarily maintained as otherwise the oxidation continues and only the end products of the reaction are obtained, namely, carbon dioxide and water.
If the reaction is carried out in the vapor phase with a solid catalyst, the reaction, taking place at the surface of the catalyst liberates the heat of the reaction at the surface of the catalyst. The transmission of the heat to the walls of the reactor is by means of the gases, the solid catalyst, or both. Since the heat capacity of the reacting gases is typically small, an inert diluent is added often to make up for this deficiency. Steam is often used because of its high specific heat. The solid catalyst, as it is usually stationary and often a poor conductor, is of little value.
However, if the reaction can be run in the liquid phase, the higher heat capacity and better heat conductivity can aid in solving the problem of temperature control. On the other hand, it is difficult to maintain good contact between a gas and a liquid, i.e., an oxgen-containing gas, oxygen, air, etc., and a hydrocarbon, especially when the gas is not very soluble in the reacting liquid. The use of a high-speed stirrer to obtain better contact between gas and liquid phases can be required. It is also necessary that the catalyst for the liquid phase operation operate at a lower temperature than is customary for the vapor phase. This requires the development of a special type of catalyst.
A number of catalysts have been developed for the liquid phase oxidation of hydrocarbons, these catalysts being typically useful for specific applications. Among these are ammonia (U.S. Pat. No. 2,632,026 for hydroperoxides), alkali and alkaline-earth metal formates, oxalates and benzoates (U.S. Pat. No. 2,681,937 -- alkyl aromatic hydroperoxides), sodium carbonate (U.S. Pat. No. 2,681,936 -- cumene hydroperoxide), the oxidative product of polycondensation of 2,4-diamino-5-phenylazotoluene (U.S.S.R. 176,874 -- cumene hydroperoxide and dimethylphenylcarbinol), a cobaltous acetate, i.e. Co(OAc).sub.2.4 H.sub.2 O (Belg. 646,849 -- phthalic acids), calcium hydroxide (U.S. Pat. No. 2,447,400 -- acetophenone and dimethylphenylcarbinol), manganese dioxide and manganese p-toluate (Senseman et al, IEC, 24 1184 (1932) -- acetophenone), manganese acetate (King et al, IEC, 21 1227 (1929) -- acetophenone).
While a large number of catalysts and procedures have been developed and disclosed for liquid phase oxidation of alkylbenzenes, the catalysts and processes are typically specific for particular products. Therefore there is a continuing need for new catalysts and new procedures with specific application in the obtaining of economically valuable products, the said catalysts being such as to permit their use in liquid phase oxidations of hydrocarbons.